Vision correction lenses, i.e., both contact lenses and intraocular lenses, must be mechanically stable, optically clear, wettable, and gas-permeable. The material that makes up the lenses must be optically clear, mechanically stable and formable to a shape which can provide sufficient correction for the eye and can maintain the correction. The lens material must be wettable since a non-wettable lens tends to irritate and cause abrasion of the eye and lid. The lens material must be gas permeable. The cell covering (corneal epithelium) of the eye respires by exchanging oxygen, carbon dioxide, and other substances with tear fluid. The placement of a contact lens over the cornea can prevent the corneal cells from contacting tear fluid and can result in oxygen starvation, build-up of carbon dioxide, discomfort and in some cases, corneal damage.
Successful hard or rigid vision correction lenses have been prepared from a variety of well known rigid polymeric substances such as glass, acrylate based polymers, styrene-based polymers and others. The polymeric nature of the materials have been rendered potentially gas-permeable using ethylenically unsaturated silicone-containing monomers that successfully provide a change in the polymeric structure of the solid lens material resulting in reduced density and increased gas permeability. Ethylenically unsaturated silicone silicone containing monomers can in general be of two types. The first type comprises a silicone group attached to the ethylenically unsaturated group through a bond that is hydrolyzable in aqueous media. The second type comprises a monomer having a bond, between the silione group and the unsaturated group, that is substantially hydrolytically stable. The art as a whole suggests that hydrolytically stable monomers be used and hydrolytically unstable monomers be avoided. Polymers having hydrolytically unstable constituents, under conditions commonly encountered by the lens, have not been used and have been actively avoided by persons skilled in the vision correction lens art since the hydrolysis of the silicone monomers is believed to result in the mechanical instability of the lens. A contact lens having mechanical instability could be easily damaged during shaping or handling or could change in dimension during wearing and alter the vision correction, rendering the lens useless for the individual. See, for example, Deichert, U.S. Pat. No. 4,341,889, and others.